Ethanol Preparation and Dehydration of Alcohols

State some uses of ethanol.

• Alcoholic drinks

• Solvent

• Fuel

What are the 2 methods of manufacturing ethanol?

1. Alkene hydration

2. Fermentation

How is ethanol produced from the alkene, ethene?

Ethene reacts with steam, producing ethanol.

C₂H₄ (g) + H₂O (g) ⇄ C₂H₅OH ΔH = -45 kJ mol-1

What are the conditions needed for the industrial preparation of alcohols?

• A temperature of 300°C

• A pressure of 60-70 atm

• A phosphoric acid (H₃PO₄) catalyst

Explain the conditions needed for the industrial preparation of ethanol, using Le Chatalier’s Principle.

Temperature

The forward reaction is exothermic, so yield would be greatest at low temperature. Because this gives a slow rate of reaction, a compromise temperature of 300°C is used.

Pressure

Two moles of reactants give one mole of products, so a higher pressure will give a higher yield. This will also increase the rate of a reaction, but remember that high pressures are expensive to maintain.

Catalyst

The catalyst is an inert solid coated with phosphoric acid. It increases the rate of a reaction, but doesn’t affect the yield.

These conditions convert around 5% of reactants to products, so the unreacted ethane is recycled back into the reaction chamber.

What is fermentation?

Fermentation is an enzyme catalysed reaction that converts sugar to ethanol.

The enzyme that catalyses the reaction is in yeast, so sugar is dissolved in water before the yeast is added. It is then left in a warm place until ethanol is formed.

C6H12O6 (aq)→ 2C2H5OH (aq) + 2CO2 (g)

How is ethanol separated from the remaining mixture?

By fractional distillation- ethanol has a lower boiling temperature than water (78°C).

What is a biofuel?

A fuel that has been produced from a biological source.

Examples include:

• Bioethanol- produced by the fermentation of sugars in plants.

• Biodiesel- produced from oils and fats present in the seeds of some plants.

State some benefits / drawbacks of producing ethanol using alkene hydration.

Benefits

• It is a continuous process ∴ more efficient.

• The rate of reaction is very rapid.

Drawbacks

• Expensive as it uses high temperatures and pressures.

• Uses finite resources based on crude oil ∴ non-renewable.

State some benefits / drawbacks of producing ethanol using fermentation.

Benefits

• Cheaper- uses gentle temperatures and atmospheric pressure.

• Renewable- the plants that produce biofuels can be grown every year. Waste materials from animals are also used.

• Carbon-neutral- Biofuels do produce carbon dioxide, but the plants that make them have taken in carbon dioxide during photosynthesis, making them carbon neutral.

Drawbacks

• Land use- land being used for crops to create biofuels cannot be used for food crops. Also, forests are being destroyed to create land on which to grow plants for biofuels.

• It is a batch process- less efficient.

Alcohols can be dehydrated to form…

Alkenes.

Dehydration of alcohols is an example of what type of reaction?

Elimination reaction- a small molecule (water) is removed from a larger parent molecule.

Give examples of dehydrating agents.

• Sulfuric acid

• Heated aluminium oxide